1. Field of the Invention
This invention relates generally to porous membranes for the controlled delivery of volatile compounds, and more specifically to a microporous membrane where the pores are non-liquid filled.
2. Description of Prior Art
There are many honeybee diseases that are treated by utilizing volatile compounds. A common use of volatile compounds is in the treatment of honeybees for parasitic mites. In the recent history of the United States bee industry these mites have had devastating effects on both commercial and hobby beekeeping. The two common mites are the tracheal mite (Acarapis woodi) and the Varroa destructor body mite (Varroa jacobsoni). The effects of the mites include the reduction of the bee population in the short term, and ultimately the death of the entire colony.
There are currently several treatments that can be used to control the Varroa mite. These include the use of Tau-fluvalinate. This is applied using a plastic strip which contains the fluvalinate. The bees come in contact with the strip and the fluvalinate kills the mites. This is marketed as Apistan(copyright) by Wellmark International. Currently, the mites have begun to show resistance to this treatment in many parts of the US and the world. Because of this, a second treatment has been approved by USEPA under section 18 of FIFRA for emergency use in some states. This is an organophosphate ( O,O-diethyl O-3-chloro-4-methyl-2-oxo-2H-1 -benzopyran-7-yl phosphorothioate ) and it is applied in a similar manner as the fluvalinate. Bayer markets this organophosphate as CheckMite+(copyright). Neither of these treats both the tracheal mite and the Varroa mite.
There is a desire to use alternates to the above pesticides. These alternates include essential oils and formic acid. The formic acid is effective as a vapor but delivery requires a highly concentrated liquid 65% v/v or greater formic acid, which is hazardous to handle because of the corrosive nature of this liquid. In addition, the delivery of the formic acid is not done in a controlled release fashion except by one method, which uses a gel formulation. The gel formulation improves the handling and delivery but does not completely eliminate the need to handle the substance. The formulation was on the market for a short time but has not been sold recently. A gel formulation containing an essential oil, thymol, is sold in Italy to control both tracheal and Varroa mites. It is currently under review for use in the United States but is not currently approved.
The challenge for any miticide that depends on the volatile nature of the compound is the control of the concentration of the volatile compound in the hive. If the compound vapor concentration exceeds a certain level the bees could be adversely affected or the bees could even leave the colony. Second, levels of the compounds in the honey could be excessive which would adversely affect the use of the honey in the market place.
Many methods have been reported in Europe and Canada for the delivery of formic acid. These methods involve the soaking of an absorbent with the formic acid and placing the absorbent into the hive. Other methods use wicks that are placed into the formic acid. All of these methods require the handling of the liquid formic acid.
Furthermore, these absorbents or wick methods do not release the formic acid at a constant rate but in general a large amount is released at the beginning and then it falls off exponentially with time. The gel formulation mentioned previously does not require direct handling of the formic acid but still requires the exposure of the gel and thus the possibility of contact with the gel containing the formic acid.
There has been a report of the use of a membrane-barrier (a nonporous membrane) in the delivery of formic acid. This is a solid membrane and must control the release by the transport of formic acid through the nonporous material. The transport through the nonporous membrane depends on the solubility of the formic acid in the membrane material and the thickness of the material.
U.S. Pat. No. 6,110,481 to Sirkar et al. teaches a controlled delivery device based on aqueous-organic partitioning in porous membranes. The pores of the membrane are filled with water or an organic liquid. The liquid in the micropores has several purposes: 1- to be highly selective;2- control the diffusion process by the type of liquid used in the pores. The process is more like an extraction process where chemical interactions of the liquid in the pore and the solute is important and results in high selectivity. The solute in the solution on the reservoir side dissolves in the liquid in the micropores and is transported through the liquid by diffusion to the other side of the membrane to be delivered to the target. Such a membrane is expensive to produce, and the rate of delivery of the compound through such is too slow to be useful in the application described above, of treating a beehive for mites.
Thus, it is an object of the present invention to develop a method that will consistently deliver the effective dose of volatile compound under many different temperature conditions.
It is a further object of the present invention to provide a delivery system for such volatile compounds that minimizes or entirely alleviates the need to handle the compounds by beekeepers and the like.
It is yet a further object of the present invention to provide a delivery system which provides a high, constant delivery rate and does not require a liquid-filled membrane.
With the above objects in mind, the present invention uses hollow, microporous membranes to deliver volatile compounds. The use of microporous membranes will address the concern of handling the compounds. In most cases, exposure to the compound may be avoided altogether after manufacturing. Of course, if the container holding the volatile compound is advertently punctured, there would be some risk of exposure. It will also deliver the vapor at a controlled rate so that the concentration is at an effective level.
In addition, microporous membranes can be made of material that can withstand the highly corrosive nature of compounds such as formic acid. The microporous membrane is typically made of a polymer that has micro channels within the polymeric material. These membranes can be produced so that they are highly uniform in pore size. This gives a high degree of control on the amount of material that can be transported through the membrane. The preferred polymer material is polypropylene, which provides a membrane that is hydrophobic and resistant to passing liquid water and large particles. The high surface tension between the membrane material and the highly polar solution prevents the solution from filling the pores. Thus a formic acid solution greater than 65% v/v would resist filling the pores of the membranes with liquid whereas the vapor would pass.
The porosity of the membrane can range from 20% to 41%, or in some cases even greater. The amount of the vapor emitted depends on the surface area and the porosity of the membrane. In addition, the amount emitted depends on the temperature of the membrane. A higher temperature will cause a higher emission rate. This means that the membranes can be designed to emit a certain amount of compound during different seasons of the year by adjusting the surface area and the porosity. For example, during the heat of summer the surface area of a 41% porosity membrane would be less than that of a fall application when the temperature is lower.
The method of delivery works by first placing the amount of compound needed for a treatment into a container which is suitable for the compound. The state of the compound can be liquid, gel or solid. The liquid, gel or solid must be in contact with the membrane in order to maximize the emission rate. In the case of formic acid the container could be made of polypropylene, polystyrene, or polyethylene. The membrane with the correct surface area for the temperature and emission rate is attached to the container and the container filled and placed in the location where the volatile compound is to be delivered, such as inside a hive. In practice the container would be filled and the membranes covered so no emission would occur. The container would then be shipped in this manner, for instance, to the beekeeper. The beekeeper would remove the covering over the membrane and place the container into the hive. This would minimize contact with the compounds and thereby minimize the exposure of the beekeeper to the compounds.
In contrast to the prior art method of using a nonporous membrane, the pores of the microporous membrane of the present invention are filled with gas. The formic acid plus water vapor is transported through the air in the pores by diffusion and the liquid material does not fill the pore because of the high surface tension difference between the liquid and the membrane material. The diffusion is faster through the gas filled pores as compared to a nonporous membrane where the diffusion takes place within the membrane material. The material surrounding the micro pores (polypropylene for example) is impermeable to the formic acid and water. This faster diffusion in the micro pores allows for a substantial reduction in surface area (approximately 4 square inches as compared to the non porous membranes which uses greater than 100 square inches). The large surface area of the nonporous membrane is a limitation in that the entire container must be composed of a thin nonporous polymer. This means that the possibility for puncturing the membrane is decreased in the present invention as compared to nonporous membranes.
When compared to the liquid-filled porous membrane of Sikkar, the present gas-filled microporous membrane provides a delivery rate a few orders of magnitude greater. Thus, the present invention is usable for delivery of formic acid into a beehive for the treatment of mites, whereas Sikkar is not.